There are various driver assistance systems for supporting a driver of a vehicle during different driving maneuvers. So-called parking assistance systems, which support the driver of the vehicle during parking, have become particularly successful. These support the driver of the vehicle to the extent that, in response to the approach to obstacles, an acoustical and/or optical warning takes place. For this purpose, parking assistance systems generally have distance sensors which are situated in the front region and in the rear section of the vehicle. Ultrasonic sensors are used as distance sensors, for example.
In order to record the surroundings of the vehicle, and therewith the distance from obstacles, the ultrasonic sensors each emit an acoustical pulse and receive the echo of the acoustical pulse. From the running time between emitting the signal and receiving the echo, the distance from an object reflecting the sound is calculated. This distance is usually shown to the driver via a color-coded optical display, for instance by a plurality of LED's having a different size and/or color, or acoustically by an interval tone having a certain frequency and/or intensity. Combinations of optical and acoustical indications are also customary. During an acoustical representation, the frequency of the interval tone and, if necessary, simultaneously the volume usually increase with decreasing distance from the obstacle. When a distance range that is critical for vehicle navigation is undershot, a stop warning to the driver takes place as a rule in the form of a continuous tone.
When ultrasonic sensors are used, the signal is sent in the form of a so-called sonic cone, which comes about from the transmission angle of the signal. However, because of the sonic cone, low obstacles such as flower tubs cannot be seen at every distance, since, with decreasing distance, they move into the shadow of the ultrasound. Particularly in the case of high sensor installation positions, low objects may dive through under the sonic cone without getting into the full warning range, and thus without generating a stop warning.
One system for obstacle detection is discussed in U.S. Pat. No. 5,574,426, for example.
Besides flower tubs and other similar objects, curbs are also detected as obstacles. But they usually do not represent a direct obstacle, because one may drive over them without their causing damage to the vehicle. However, in the case of parallel parking spaces, since the lateral boundary is usually formed by curbs, in parking assistance systems, which support the driver during parking in such spaces, it is necessary to detect the curb as a lateral boundary.
In semiautomatic or fully automatic parking assistance systems, in which the parking space is first measured while driving past, a parking trajectory corresponding to the parking space geometry is calculated, and steering instructions for driving into the parking space are given to the driver, or steering interventions are taken over fully automatically, a curb, which represents a low boundary, is recorded as a boundary of the parking space. In the known systems, low and high boundaries are evaluated equally, and the parking trajectory is calculated so that driving over the boundary does not occur. In the case of a low boundary, for instance a curb, whose height is so low that the vehicle is able to tower over it, the trajectory could, however, be calculated differently, since no collision is produced by driving over the low boundary. In the known systems, since the height of a bounding object is not taken into consideration, the optimal parking trajectory therefore cannot be calculated in the known systems for parking spaces having low boundaries. In addition, in a parking space having a low boundary, for instance, a curb, the vehicle is able to be parked closer to the boundary.